The present invention relates to a method and a tool for producing centered parts having spherical surfaces.
The present invention has particular application to a method and a tool for producing optional parts.
In certain applications it is important to have curved surfaces of optical parts accurately formed and aligned without the use of a costly centering operation.
Spherical surfaces for optical parts have been produced in the prior art by methods and tools which included a block for supporting the parts during the grinding operation.
In the prior art two techniques have been used for supporting the parts on the block.
In one technique, spot blocking, holes were bored into the block through its spherical surface. In this technique all holes must be bored so the bottom of the holes are tangent to the same sphere, and the angular alignment of the bored hole with respect to the center of curvature of the spherical surface can be critical. Since the inner ends of the parts are supported on the milled surface at the bottom of the bore, any variations of the milled surface from true tangency resulted in a lack of centration of the spherical surface ground on the related part. Any internal chamfer between the sidewall of the hole and the milled surface could cause cocking of the part; and any play between the sidewall of the hole and the part could permit a lateral shift which could defeat concentricity of the outer surface of the part. Supporting the ends of the parts on the surfaces of milled holes thus required all the holes to be drilled with great accuracy to the same depth, at the proper angular alignment with respect to the center of curvature of the spherical surface on the block, without internal chamfers between the milled surface and sidewalls, and without slack between the sidewalls and the parts in order for all the spherical surfaces ground on the parts to have centered spherical surfaces. In practice it was difficult to approach an accuracy of fifteen minutes of arc with this prior art technique, and the accuracy actually obtained has typically been in the order of thirty minutes of arc.
In another prior art, flood blocking, technique the parts were attached to the spherical surface of the block by wax or by another substance, rather than being positioned in holes drilled through the spherical surface of the block. This technique did not always provide the desired degree of lateral restraint to permit the parts to be ground and polished vigorously at high rates of production. This technique also presented problems of shrinkage of the wax or other substance as the wax or other substance cooled; and this could cause misalignment of the parts. Also, since the flood blocking technique used a lay-in tool to position the parts on the block, any error on the unfinished end of the part could be transferred to the finished end of the part.